Method for producing directionally solidified cast alloy articles

ABSTRACT

A method of producing directionally solidified cast alloy articles is disclosed wherein a porous, shell mold having openended mold cavities is filled with molten alloy while seated on a low heat conductive surface and is allowed to cool. The filled mold is then moved to a highly heat conductive surface and is reheated to a temperature above the melting range of the alloy where a unidirectional temperature gradient is established along the length of the alloy within the mold to cause directional solidification of the alloy.

limited States Patent 1191 Mullen Aug. 28, 1973 [5 METHOD FOR PRODUCING 3,441,078 4/1969 Chandley 164/53 DIRECTION ALLY SOLIDIFIED CAST 3,667,533 6/1972 Boucher et a]. 164/60 ALLOY ARTICLES FOREIGN PATENTS OR APPLICATIONS [75] Inventor: Richard M. Mullen, Indianapolis, 543,809 1/ 1932 Germany 164/80 Ind.

Primary Examiner-J. Spencer Overholser [73] Asslg'nee' 3:1 :53 22:28 Corporation Assistant Examiner-John E. Roethel Attorney-Sidney Carter [22] Filed: Feb. 15, 1972 [21] Appl. No.: 226,491 [57] ABSTRACT A method of producing directionally solidified cast 52 us. on 164/60, 164/80, 164/127, alloy articles is disclosed wherein Porous, mold 164/338 having open-ended mold cavities is filled with molten [51] int. Cl 822d 27/06 While seated 810W heal conductive surface and 58 Field 61 Search 164/60, 80, 122, is allowed to COOL The filled mold is then moved to 164/123 125, 338 highly heat conductive surface and is reheated to a temperature above the melting range of the alloy where [56] References Cited a unidirectional temperature gradient is established UNITED STATES PATENTS along the length of the alloy within the mold to cause directional solidification of the alloy. 2,951,272 9/1960 Kiesler 164/80 3,204,301 9/1965 Flemings et aI 164/53 5 Claims, 3 Drawing Figures R 10 N 5 a 50/3 a 4. 2 a Q S I S as ,ygw, 1 13s 5 Q A 1 2 g I Z2 [626 E //s W' k l5 I6 13 Z PATENTEU AUS 2 8 I975 METHOD FOR PRODUCING DIRECTIONALLY SOLIDIFIED CAST ALLOY ARTICLES The invention herein described was made in the course of work under a contract or subcontract thereunder with the United States Air Force.

This invention relates to a method of casting high temperature alloys and, more particularly, to a method for casting directionally solidified alloy articles, such as gas turbine engine blades and vanes.

Gas turbine engine blades and vanes are subject to high temperatures and stresses and to extreme thermal cycling. Recent studies have shown, however, that blades and vanes having a directionally oriented columnar structure exhibit improved high temperature properties over blades and vanes having an equiaxed grain structure, particularly in fracture resistance and ductility under creep loading conditions. In forming directionally solidified columnar grain castings by current methods, an open-ended mold having a highly heat conductive chill plate secured to the base thereof is heated to establish a unidirectional temperature gradient along its length and is then filled with molten metal. For example, when casting alloys having a melting point at about 2,400 F, the mold is heated to establish a temperature near the chill plate of about 2,000 F, while in the upper portions of the mold remote from the chill plate, the temperature is about 2,700" F. As a result of the unidirectional temperature gradient and the chill at the base of the mold, crystals growing in the melt form with their preferred crystallographic orientation, the (100) orientation for body centered and face centered cubic systems, for example, substantially parallel to the direction of the thermal gradient and grow in a direction away from the chill plate, resulting in columnar grain growth from one end of the mold to the other.

One of the disadvantages of forming columnar grain castings, however, lies in the fact that many of the high temperature alloys used in forming gas turbine engine blades and vanes are not capable of being cast in air. Accordingly, the casting procedure must be carried out in a vacuum furnace thus requiring expensive vacuum melting and pouring equipment. That is, the alloy must be melted and poured into the preheated mold in the vacuum chamber and allowed to solidify therein. Depending on the thermal cycling required to produce the oriented columnar grain structure, this procedure may take anywhere from 30 minutes to several hours. It is apparent then that current procedures for forming columnar grain castings require relatively expensive equipment which must be used for a relatively long period of time per casting.

In accordance with the present invention, a method of producing directionally solidified cast alloy articles is provided which includes filling a, mold, usually of the porous shell mold type having an open-ended mold cavity, with the alloy in molten form, allowing the mold to cool for a short period of time whereby the metal in the mold cavity may either completely solidify or preferably cool sufficiently. to form a self-sustaining skin layer, and then bringing a highly heat conductive chill plate into contact with the mold at the open end of the mold cavity with the chill plate contacting the alloy in the mold cavity. The filled mold is then reheated to either completely remelt the solidified alloy within the mold cavity or reheat the cooled metal and to establish a unidirectional temperature differential along the length of the alloy in the mold cavity with the tempera ture of the alloy on heating being at all points along its length above its melting point and thereafter with the temperature being progressively greater in the direction away from the chill plate. The directionally heated mold is then allowed to cool to cause directional solidification of the alloy within the mold cavity in a direction away from the chill plate.

When casting alloys requiring vacuum casting techniques, those steps of the process requiring a vacuum environment are only melting of the alloy and filling of the mold with the alloy. On completion of these steps the filled mold may be removed from the vacuum environment and the reheating and cooling steps performed in an atmospheric environment to cause directional solidification of the alloy within the mold cavity. As a result, the vacuum melting and pouring equipment is used for only several minutes per casting after which a new mold may be positioned in the vacuum chamber for casting another article thereby reducing the vacuum equipment cycle time and achieving higher production rates.

Other advantages of this invention will be apparent from the following detailed description, reference being had to the accompanying drawings of which:

FIG. 1 is a cross-sectional view of a multiple cavity porous shell mold used in practicing this invention;

FIG. 2 is a cross-sectional view of the shell mold shown in FIG. I mounted in a tapered flask immediately before filling with the molten alloy; and

FIG. 3 is a cross-sectional view of the shell mold assembly shown in FIG. 2 with the flask filled with an exothermic material and the mold filled with metal.

Referring now to FIG. 1, there is shown a porous ceramic shell mold 10, formed by the well-known lostwax method, having multiple mold cavities 12 of the shape of the article to be cast. The mold cavities 12 have an open end 13 at the bottom of the mold l0 and communicate with the mold riser 14 through a gating system 16. This gating system allows any particles washed out of the shell mold to travel through the mold cavities and into the riser 14 thus improving the quality of the cast articles. The gating system 16 has air space cavities 18 between the bottom surface of the mold and the open end 13 of the mold cavities 12 for improved heat extraction from the mold cavities 12, as will hereinafter be more fully explained.

Referring now to FIG. 2, the porous ceramic shell mold 10 is preheated to approximately 2,000 F and placed in a tapered flask 20 preferably formed of a heat resistant material. The flask 20 includes a flange 22 in the bottom thereof to provide support for the ceramic shell mold 10 during handling. The flask 20 is also lined with a protective material 23, such as a sheet of ashestos. The heated shell mold 10 within the flask 20 is placed in a vacuum chamber resting on a pouring plate 24 made of a low thermal conductivity material and is filled with molten alloy in the range of 200 F to 400 F above the melting point of the alloy. The poured alloy is then allowed to cool for a short period of time and is then moved to a highly heat conductive surface such as a copper chill plate 26 having a circulating coolant, such as water, therethrough to achieve maximum chilling effect. The mold 10 after pouring may be allowed to cool until the alloy therein is completely solidified, however, in open bottom molds, as shown in FIG. 2, it is only necessary that the mold be sufficiently cooled for the formation of a self-sustaining skin layer across the open end 13 to prevent run-out of the partially still molten alloy. When the mold is filled as described above, sufficient cooling takes place for movement of the mold to the chill plate in only several minutes. When casting in a vacuum environment, once the mold has been removed another mold can then be placed in the vacuum chamber and filled. It is apparent that the short cooling time involved in the vacuum environment allows for more efficient use of the vacuum melting and pouring equipment.

On positioning of the mold on the chill plate 26 the open end 13 of the mold cavities 12 contacts the surface of the chill plate. However, as shown in FIG. 3, the air space cavities l8 prevent contact between the bottom surface of the mold and the chill plate 26. These cavities 18 function as a heat extraction barrier between the mold 10 and the chill plate 26 thereby minimizing heat loss from the mold for improved heat extraction of the chill plate with regard to the alloy in the mold cavities 12 in contact with the chill plate 26 at the open end 13 of the mold. This improved heat extraction increases the solidification rate of the alloy in the desired direction thus improving the dendrite arm spacing within the cast article, especially in regions remote from the chill plate.

The flask is then filled with high density pebbles 28 made of an exothermic material. The pebbles are then ignited and allowed to burn to completion where upon the mold is rapidly heated to establish a unidirectional differential temperature along the length of the alloy within the mold cavities 12. The flask 20 is tapered from large at the top to small at the bottom. This geometric shape along with the use of the exothermic pebbles 28 allows the establishment of a proper thermal gradient within the mold 10 which has heretofore been necessarily established by using layers of exothermic materials of varied thermal outputs. These exotherinic pebbles are typically formed of aluminum and iron base materials with various fillers and binders. No mixing, ramming, or baking of the material is required in the preparation of the mold. Furthermore, the pebbles have a characteristic burning temperature and thus, since the mold temperature is determined by the geometry of the flask, the need for complicated and constant mold temperature monitoring systems is eliminated.

Because of the tapered flask 20 and the chill plate 26, after burning of the exothermic material the mold 10 is at a higher temperature in the regions remote from the chill plate 26 and accordingly, a temperature gradient of approximately 200 F is established along the length of the alloy within the mold very shortly after burning. This temperature is sufficient to remelt the cast article and reheat the alloy and establish directional solidification of the alloy upwardly from the chill plate 26. As soon as the exothermic material has burned to completion, the top of the material is covered with a pad of insulation 29. The mold is then allowed to cool to cause directional solidification of the alloy upwardly in a direction away from the chill plate.

Upon subsequent cooling of the cast article, the article is removed from the mold and finished in accordance with normal finishing procedures.

If desired, to achieve optimum surface finish in the resultant cast article about 10 grams per pound of metal of hexamethylenetetramine may be added to the hot riser. The entire mold assembly may then be cov cred with an air'tight container 30 wherein the hexamethylenetetramine establishes a reducing atmosphere which produces a cast article with an improved cast surface. Argon or other inert gases may be similarly used.

Reference to the following specific example will further serve to illustrate my invention and the advantages thereof.

EXAMPLE Standard test bars were cast in a porous shell mold of the type shown in FIG. 1. The articles were cast from Mar-M-246 alloy which has the following nominal composition, by weight: 0.15% carbon, 0.l0% manganese, 0.05% silicon, 9.0% chromium, 10.0% cobalt, 2.5% molybdenum, 10.0% tungsten, 1.5% titanium, 5.5% aluminum, 0.015% boron, 0.05% zirconium, 0.15% maximum iron, balance nickel.

The mold was preheated to a temperature of about 2,000 F and placed in a vacuum melting and pouring apparatus. The molten alloy was poured at a temperature of 2,900 F. The filled mold was then removed from the vacuum chamber within two minutes after pouring and placed on a water-cooled copper chill plate. The tapered sleeve surrounding the mold was then filled with pebbles of exothermic material ob tained from Exomet, lnc. of Conneaut, Ohio. The pebbles were H41 inch long, /5 inch wide and inch thick. The pebbles were then ignited and allowed to burn to completion heating the mold and the alloy therein to about 3,100 F. Within about 3 minutes the temperature at the base of the mold dropped to about 2,900 F thereby establishing a 200 F temperature gradient along the length of the alloy within the mold cavity. The mold then was allowed to cool causing directional solidification of the alloy upwardly from the chill plate. The mold was allowed to cool for about 40 minutes after which time the cast article was removed from the mold. Examination of the article revealed an excellent directionally oriented columnar grain structure.

Although my invention has been described in terms of a specific embodiment it will be recognized that various modifications and other forms may be adopted within the scope of my invention. For example, al though my invention has been described in terms of the use of exothermic pebbles, it will be recognized that other heating means, e.g., Calrod electrical heaters, tubular exothermic sleeves, and the like, may be used to establish the thermal gradient. Further, although my invention has been described in terms of forming an article completely of a columnar structure, it will be recognized that the heating means may be arranged to remelt or reheat only a portion of the alloy within the mold cavity. Accordingly, if desired, turbine blades having an equiaxed grain root portion with a columnar grain shroud portion may be conveniently formed by my invention. Still further, it will be recognized that my invention is equally applicable whether applied to molds open at the bottom wherein grain growth proceeds upwardly from the chill plate, or to molds open at the top wherein grain growth would proceed downwardly from a chill plate contacting the top of the mold.

1 claim:

1. A method of producing a directionally solidified cast alloy article in a mold having a vertically extending mold cavity therein open at one end, said method comprising filling said mold with the alloy at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, bringing a highly heat conductive chill plate into contact with said mold at said open end with the alloy in said mold cavity at said open end contacting said chill plate, reheating said mold with said alloy therein to a temperature above the melting range of the alloy, and allowing said mold to cool to establish directional solidification of the alloy in said mold cavity in a direction away from said chill plate.

2. A method of producing a directionally solidified cast alloy article in a ceramic shell mold having a vertically extending mold cavity therein open at the bottom thereof, said method comprising filling said mold seated on a low heat conductive surface with the alloy at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, thereafter seating said mold on a highly heat conductive chill plate with the alloy in said mold cavity at said open end contacting the surface of said chill plate, reheating said mold with said alloy therein to a temperature above the melting range of the alloy, and allowing said mold to cool to establish directional solidification of the alloy in said mold cavity upwardly from said chill plate.

3. A method of producing directionally solidified cast alloy articles in a porous, ceramic shell mold having multiple mold cavities therein open at the bottom thereof, said method comprising filling said mold seated on a low heat conductive surface with the alloy at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, thereafter seating the open ends of said mold on a highly heat conductive surface with the alloy in said mold cavities at said open ends contacting the surface of said chill plate, reheating said mold with said alloy therein to a temperature above the melting range of the alloy, and allowing said mold to cool to establish directional solidification of the alloy in said mold cavities upwardly from said chill plate.

4. A method of producing a directionally solidified cast alloy article in a mold having a vertically extending mold cavity therein open at the bottom thereof, said method comprising filling said mold seated on a low heat conductive surface with the alloy at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, thereafter seating said mold on a highly heat conductive chill plate with the alloy in said mold cavity at said open end contacting the surface of said chill plate, positioning a tapered, heat resistant flask about said mold, said flask extending from said chill plate to the top of said mold and having a progressively increasing diameter in a direction away from said chill plate, filling said flask with a preformed exothermic composition having a characteristic burning temperature above the melting range of the alloy, igniting said composition while so positioned, and thereafter allowing said mold to cool to establish directional solidification of the alloy in said mold cavity upwardly from said chill plate.

5. A method of producing a directionally solidified cast alloy article in a mold having a vertically extending mold cavity therein open at one end, said method comprising filling said mold with the alloy in a vacuum environment at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, removing said mold from the vacuum environment, bringing a highly heat conductive chill plate into contact with said mold at said open end with the alloy in said open end contacting said chill plate, reheating said mold with said alloy therein to a temperature above the melting range of the alloy, and allowing said mold to cool to establish directional solidification of the alloy in said mold cavity. 

2. A method of producing a directionally solidified cast alloy article in a ceramic shell mold having a vertically extending mold cavity therein open at the bottom thereof, said method comprising filling said mold seated on a low heat conductive surface with the alloy at a temperAture above the melting range of the alloy, said alloy being operative to heat said mold, thereafter seating said mold on a highly heat conductive chill plate with the alloy in said mold cavity at said open end contacting the surface of said chill plate, reheating said mold with said alloy therein to a temperature above the melting range of the alloy, and allowing said mold to cool to establish directional solidification of the alloy in said mold cavity upwardly from said chill plate.
 3. A method of producing directionally solidified cast alloy articles in a porous, ceramic shell mold having multiple mold cavities therein open at the bottom thereof, said method comprising filling said mold seated on a low heat conductive surface with the alloy at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, thereafter seating the open ends of said mold on a highly heat conductive surface with the alloy in said mold cavities at said open ends contacting the surface of said chill plate, reheating said mold with said alloy therein to a temperature above the melting range of the alloy, and allowing said mold to cool to establish directional solidification of the alloy in said mold cavities upwardly from said chill plate.
 4. A method of producing a directionally solidified cast alloy article in a mold having a vertically extending mold cavity therein open at the bottom thereof, said method comprising filling said mold seated on a low heat conductive surface with the alloy at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, thereafter seating said mold on a highly heat conductive chill plate with the alloy in said mold cavity at said open end contacting the surface of said chill plate, positioning a tapered, heat resistant flask about said mold, said flask extending from said chill plate to the top of said mold and having a progressively increasing diameter in a direction away from said chill plate, filling said flask with a preformed exothermic composition having a characteristic burning temperature above the melting range of the alloy, igniting said composition while so positioned, and thereafter allowing said mold to cool to establish directional solidification of the alloy in said mold cavity upwardly from said chill plate.
 5. A method of producing a directionally solidified cast alloy article in a mold having a vertically extending mold cavity therein open at one end, said method comprising filling said mold with the alloy in a vacuum environment at a temperature above the melting range of the alloy, said alloy being operative to heat said mold, removing said mold from the vacuum environment, bringing a highly heat conductive chill plate into contact with said mold at said open end with the alloy in said open end contacting said chill plate, reheating said mold with said alloy therein to a temperature above the melting range of the alloy, and allowing said mold to cool to establish directional solidification of the alloy in said mold cavity. 